Mechanical ventilators are regularly used in the treatment of medical patients who suffer from impaired respiratory function or respiratory failure. Respiratory impairment or failure may result from inadequate ventilatory muscles, chest injury, and lung disease, as well as from a variety of cardiac, neurological, and neuromuscular disorders. In severe cases, a mechanical ventilator is used to completely take over the patient's breathing function by alternately forcing inflation gas into the lungs and thereafter removing exhalation gas from the lungs, thereby regulating the exchange rate of gases in the blood.
Mechanical ventilators typically aspirate at relatively high air volumes and pressures. Recent studies have shown, however, that such use of high air volumes in mechanical ventilators may subject the patient to a number of ventilator-induced lung injuries or volutraumas. In order to lessen the occurrence of these ventilator-induced volutraumas, and for other reasons, a lung protection technique has been utilized which limits the peak alveolar pressure and tidal volume in the lungs, and allows higher concentrations of carbon dioxide in the blood. While this ventilation technique, called permissive hypercapnia, may be tolerated by many patients, it can cause complications in other patients due to acute acidosis, or high acidity in the blood.
One method of decreasing complications due to acute acidosis during permissive hypercapnia involves increasing the removal of carbon dioxide during ventilation by supplying a secondary source of air into the trachea at a higher velocity but lower volume than the mechanical ventilator. This procedure, called tracheal gas insufflation (TGI), creates turbulent airflow in the lungs during exhalation. The turbulence in the lungs increases the amount of carbon dioxide being carried from the lungs with each ventilator induced exhalation, and thereby reduces the acidity in the blood. The turbulence prevents the formation of pockets or dead space volumes of carbon dioxide in the lungs, which might otherwise occur using only the mechanical ventilator for respiration.
TGI is typically accomplished by inserting a gas catheter into the patient's endotracheal tube, and ideally positioning the distal tip of the TGI catheter one to two centimeters from the patient's carina where a branch passage to each lung occurs. This positioning may result in the TGI catheter extending beyond the distal end of the endotracheal tube. An air and oxygen mixture, usually identical to the gas mixture used in the mechanical ventilator, is then introduced into the trachea via the TGI catheter, to cause the turbulence which results in more effective carbon dioxide removal.
One concern associated with both ventilation and TGI is the potential for an occlusion of the endotracheal tube. An occlusion can be particularly dangerous if the endotracheal tube becomes occluded with the tip of TGI catheter positioned beyond the end of the endotracheal tube. When this occurs the gas emitted from the TGI catheter does not have a pathway to escape from the patient's lungs. The continued delivery of TGI gas under such conditions may result in lung tissue damage or a rupture of one or both lungs.
A common cause of occlusions is the accumulation of mucus in the endotracheal tube, although other types of physiological or mechanical occlusions might also occur. To prevent excessive drying of the tissue, damage to the patient's mucus membranes and the accumulation of mucus which might possibly lead to an occlusion, it is desirable to heat and humidify the TGI airoxygen mixture. The heated and humidified gas mixture helps prevent an occlusion from the accumulation of mucus. Mechanical ventilators typically humidify, filter, and warm the inspired gases to avoid or lessen the occurrence of mucus. It is also desirable to heat and humidify the TGI gas mixture, but certain difficulties occur in TGI therapy as a result of heating and humidifying the TGI gas mixture.
Stopping the flow of heated and humidified TGI gas in response to endotracheal tube occlusion or an unexpected pressure increase in the patient's lungs presents a number of problems in TGI therapy. First, a typical TGI heat and humidification device employs a relatively long, electric heating element or wire to heat the humidified gas. The humidified gas passes over the heating element which runs internally along a length of gas hose supplying the TGI catheter. If the gas flow through the hose is stopped, overheating and damage to the hose can occur because the terminated air flow is no longer available to remove residual heat from the heating element. Even a temporary interruption in the gas flow will cause a rise in temperature in the TGI gas, requiring a resumption of the airflow for a relatively lengthy time to reestablish the proper gas temperature operating conditions before the TGI gas can be again delivered to the patient. Second, simply shutting off the heat and humidification device requires the reestablishment of the desired heat and humidity levels before supplying the TGI gas to the patient. Finally, the time required to reestablish the TGI gas flow at the desired conditions after the TGI gas flow is interrupted or after the TGI delivery system is shut off precludes the immediate resumption of TGI therapy, which may induce further trauma in an already-compromised patient.
Another type of TGI therapy is referred to as intermittent or phasic TGI. Phasic TGI therapy involves supplying TGI gas to the TGI catheter and into the trachea only during a portion of the exhalation period of the respiratory cycle created by the mechanical ventilator, but not during the inhalation period of the respiratory cycle. Phasic TGI therapy is considered as offering a potential for therapeutic benefit because the resulting turbulence during exhalation enhances carbon dioxide removal. Phasic TGI also decreases the total gas volume contributed during TGI therapy and allows for simpler operation of the mechanical ventilator.
A significant disadvantage of current phasic TGI therapy is that heated and humidified gas cannot be used due to the problems associated with maintaining the desired heat and humidity levels in the delivered TGI mixture of oxygen and air, as noted above. Consequently, all known phasic delivery TGI therapies use only non-heated and non-humidified air, which may lead to the complications of an increased risk of occlusions from mucus, dried tissue, and damage to the mucus membranes, among other things.
It is with respect to these and other considerations that the present invention has evolved.